A long-term study of wild chimpanzees in Uganda has documented only the second recorded permanent group split in the species, finding that the division unfolded gradually over years before giving way to lethal intergroup conflict.
The question of how and why animal societies fracture has relevance beyond primatology. Groups are not fixed structures they shift with social relationships, resource availability, and individual decisions. Because chimpanzees are among humans’ closest living relatives, understanding how their communities divide and what follows can inform broader theories about the evolutionary roots of intergroup conflict.
Before this study, the only documented group split in wild chimpanzees came from Jane Goodall’s observations of the Kasekela community in Gombe, Tanzania, roughly 50 years ago. In that case, a subgroup broke off and many of its members were subsequently killed by their former groupmates. No comparable event had been systematically observed in wild chimpanzees since, leaving open questions about how common such splits are, what drives them, and whether the Gombe violence was representative.
Sandel and colleagues studied the Ngogo chimpanzee community in Uganda, which has been monitored continuously for nearly 30 years. The community had grown to more than 200 individuals, making it the largest habituated chimpanzee population on record. The researchers combined direct behavioral observation with social network analysis to track associations among individuals before, during, and after the community divided.
Their analysis showed the split was not sudden. Network clusters were detectable through association data prior to the full division, but many individuals moved between clusters from year to year, and the community remained a single social group. Over time, ties connecting the two emerging sides weakened. The deaths of individuals who had maintained social bonds across the forming divide lost to disease accelerated the separation. Eventually, association between the two sides ceased entirely, and the division became permanent.
What followed was not peaceful separation. Members of both groups began patrolling their new territorial boundaries. This escalated into lethal violence directed first at adult males and later at infants.
Sandel et al. reported that abundant resources, which had supported the Ngogo community’s unusually large size, did not produce more tolerant relations between the two sides after the split. The commentary author, James Brooks, notes this finding complicates at least one existing hypothesis that food abundance explains why bonobos, equally closely related to humans, tend toward tolerant intergroup relations rather than violent ones. In the Ngogo case, resource abundance appears to have enabled population growth that preceded the split, without preventing the violence that followed.
The Ngogo split is notable for an additional reason: unlike the Gombe case, there was no history of food provisioning at the research site, removing a potential confounding factor.
The researchers and commentators acknowledge several limitations. Long-term data collection at Ngogo has centered on adult males, and the role of female chimpanzees whose decisions about space use, food, social relationships, and reproduction shape group structure remains understudied. Brooks writes that territorial patrols and intergroup violence, though primarily carried out by males, are outcomes of group-level processes that cannot be fully understood without attention to both sexes. It is also premature, the commentary notes, to draw firm conclusions from a single community about what drives group cohesion or rupture, given meaningful differences between chimpanzee populations in human disturbance, predation pressure, and ecology.
Only two group splits have been documented in wild chimpanzees across decades of intensive observation across multiple long-term sites. Whether the Ngogo pattern reflects something general about chimpanzee social dynamics, or something particular to this community’s history and size, remains an open question.
References:
A. A. Sandel et al., Science 392, 216 (2026). Commentary by James Brooks, Science 392, 146 (9 April 2026).
Note: This article is based on a published commentary describing the Sandel et al. study.
